Bed cover = exta weight. Shockingly enough, they won't improve your MPG. I can't remember the thread but somebody explained it to me over on TTORA. I'll search over there later and see what I can dig up.

i don't know if you watch Mythbusters, but they did an episode on this exact thing. under controlled conditions, they tested out caps, tenneaus, tailgate down; and in the end, the only option that actually improved mileage, was when they replaced the tailgate with one of those mesh like (soft) sailgates with the wholes in it.

so the bottom line was that weight of a cover outweighed any aerodynamic gains. i got one anyways though because it keeps everything dry and locked.